Taptaptap has a great list of helpful iPhone tips and tricks that is sure to have something new for even the most seasoned of iPhone users. One tip that was a pleasant surprise in particular was this:

Tapping the status bar (the bar at the top with the clock) will make scrollable content scroll to the top. It comes in handy in situations like when you’ve scrolled down a long web page or mail message and you quickly need to get back to the top.

This may be the most convenient, yet hidden feature on the iPhone. I had my phone for months before stumbling upon it (and I didn’t even stumble upon it by playing around with the phone, but by reading the programming documentation on scroll views).